>Now Windows 8 (moreso RT, but even the x64 version) and Mountain Lion are moving to the walled-garden, segregated, impotent app approach

What can't you do in ML that you could in OS X 10.1?

If and only if the app signing and approval by Apple gets mandatory to run apps, a la iOS, then you'll have a point.

As for now, all the new signing etc are additions on top of the regular infrastructure, and the reason they were added was security.

Most security nerds find app signing and sandboxing a good idea.